What is normal?

Content

This page gives examples of 'normal' calibration frames, for quick
comparison. If calibration frames look significantly different, there
may be some problem. All images are for 2x2 binning mode, and are made
using the same skycat programme that is used for the RTD, with automatic
cuts (except for the imaging flats, where cuts between ~35k and 45k were
used to show the structure in the flat field image, and in the fringing
and 2nd order contamination images, where the cuts are selected to show
these faint structures). The cuts though the images (click on the link
below the corresponding image) are taken
vertically (along the dispersion direction for spectroscopy) through
the middle of the frame.

Bias

The bias frame is quite flat, any noticable pattern is not normal.
Only the fast mode shows different levels between the two halves, as it
uses two amplifiers to read out.

Normal

Fast

Imaging flats

The most noticable thing is the central light condensation due to
the focal reducer, and the vignetting at the corners. A horizontal line of bright
points is occasionally visible, this is due to reflections from the
back of the filter wheel, and will hopefully be remedied by the
installation of a new wheel in the near future.

Note that these images are from dome flats, but it is recommended
to take twilight sky flats for imaging. See
Imaging calibrations page for details.

U

B

V

R

g

r

i

z

Spectroscopic flats

Slight variations along the x direction are due to imperfections in
the slit - these are very small, but it is worth taking twilight
observations to make a slit illumination correction if you need
to know the differences in slit width along the slit to a high
accuracy. Note that the VPHGs introduce a shift in the light path,
so it appears that part of the image is 'missing'. Fringes are visible
for some grisms; these will shift slightly with the instrument flexures
so users requiring very good correction for fringes (those needing
to identify faint lines at red wavelengths) should also take internal
flats during the night at the same position as the science exposures.

Other

Other things to note are the fringe pattern in night time imaging
in red filters (see fringing page)
and the second order that is visible when taking spectra of bright
blue objects (eg standard stars) in some grisms (see this
report (pdf) and the advice on
reducing this with order sorting
filters).